Conventional MEMS resonators have a construction shown in FIG. 5. FIG. 5(a) is a perspective view of a main part of a conventional MEMS resonator, FIG. 5(b) is a top plan view of the MEMS resonator, and FIG. 5(c) is a sectional view thereof.
In FIG. 5, the resonator is formed of a support substrate 701, a first insulation layer 702 which is a sacrificial layer, a beam 703, a support portion 704 which supports the beam 703, a second insulation layer 705 which is a sacrificial layer and which forms a gap, and electrodes 706. The first insulation layer 702 functions as a spacer. In FIG. 5, for example, an SOI (Silicon On Insulator) substrate is used which is formed by forming a first insulation layer 702 which is made up of a thermal oxidation layer on a support substrate 701 which is made of a single-crystal silicon and affixing a single-crystal silicon layer thereto. An oxidation layer formed of a TEOS (Tetraethyl Ortho Silicate) is used for the second insulation layer 705, and a polycrystalline silicon is used for the electrodes 706.
In order to form a beam 703 and a support portion 704, a triangular prism-shaped structure is formed by using an anisotropic etching process. Next, an oxidation layer which forms a second insulation layer 705 and a polycrystalline silicon layer which forms electrodes 706 are formed. As this occurs, the second insulation layer 705 is removed between the beam 703 and the electrodes 706, so as to form a gap. Next, the electrodes 706 are etched. In this etching process, in the construction in which the triangular prism-shaped structure projects, irregularities are large, and because of this, it becomes difficult to implement a photolithography thereon. Thus, a process is necessary to form a resist to a desired height.
As one of methods for forming a resist, there is disclosed a method for forming a resist after electrodes 706 have been formed. FIG. 6 shows process flow diagrams of producing an MEMS resonator. Sectional views are shown on a left-hand side, and top plan views are shown on a right-hand side of the figure. In FIG. 6, (a) shows an SOI substrate which is formed by forming a first insulation layer 802 which is made up of a thermal oxidation layer on a support substrate 801 which is formed of a single-crystal silicon and further affixing a single-crystal silicon layer 803s to the first insulation layer 802, (b) shows a state in which a beam 803 and a support portion 804 which are made of a single-crystal silicon are formed by patterning the single-crystal silicon layer 803s through anisotropic etching, (c) shows a state in which a second insulation layer 805 is formed which is made of a TEOS and which defines a gap, (d) shows a state in which a polycrystalline silicon layer 806s is formed which configures electrodes, and (e) shows a state in which a resist 807 is formed to a desired height. Here, in the steps from (d) to (e), for example, the thickness of the projecting beam 803 is referred to as 3 μm, the thickness of the second insulation layer 805 is referred to as 0.3 μm, and the thickness of the polycrystalline silicon layer 806s for an electrode is referred to as 1.0 μm. When attempting to form electrodes 806 to a height which is half the height of the beam 803, a construction can be realized in which only an apex of the triangle is exposed by forming a resist 807 whose thickness is 2.4 to 2.5 μm and the other portions of the beam 803 is covered with the resist 807. For example, Patent Literature 1 describes an MEMS resonator as a related-art MEMS resonator like the one described above.
Alternatively, an opening 808 can also be formed in a resist 807 by forming the resist 807 to a thickness which could enable the planarization of the projecting portion of the beam 803 and applying an etch back to the resist 807 formed so as to cause only an apex of a triangle to be exposed. For example, when a resonator is formed by layers which are formed to the thicknesses described above, the thickness of the resist 807 needs to be 5 to 6 μm. Patent Literature 2 describes an MEMS resonator as a related-art MEMS resonator like the one described above.
Thereafter, in step (f), the polycrystalline silicon layer 806s is etched over the whole surface of the wafer so as to be cut to be separated into input and output electrodes, so that input and output electrodes 806 are formed. Then, in step (g), after the resist 807 is removed, the electrodes 806 are patterned through photolithography. Finally, in step (h), the first insulation layer 802 and the second insulation layer 805 are etched with fluorine in a gas phase so as to form a hollow construction, whereby the MEMS resonator is completed in which the beam 803 functions as a vibrator.